1) Field of the Invention
The present invention relates to an image forming apparatus that forms a color image by using an electrophotographic process.
2) Description of the Related Art
In a color image forming apparatus that uses the electrophotographic process, toner recycling has been strongly demanded from the viewpoint of improving energy saving and maintenance of the apparatus. However, mixing of colors becomes a hindrance in carrying out the toner recycling.
The color image forming apparatus usually forms toner images on image carriers using three colors or four colors. The toner images of the colors formed on the image carriers are then transferred onto an intermediate transfer unit or a transfer material as a transfer medium, thereby to form a full-color image. A cleaning unit is provided for each image carrier corresponding to each color. The cleaning unit recovers toner remaining on the image carrier after transfer (hereinafter, “residual toner”). The toner recovered is recycled.
However, this type of image forming apparatus has the following problem. That is, when a first color toner is transferred onto the intermediate transfer unit and a second color toner is transferred onto the intermediate transfer unit, the first color toner is reversibly transferred onto the image carrier. When this reverse transfer phenomenon occurs, colors of the recovered toners are mixed within the cleaning unit, which makes it difficult to carry out the recycling of the toners.
In the mean time, color reproducibility on the image is regarded most important. The color reproducibility is determined based on various image forming factors such as characteristics of image processing, charging, exposure, developing, transfer, fixing, toners, and a photoreceptor. When the color reproducibility is lost, it is necessary to adjust it. However, it is difficult to simply specify a cause of the loss of the color reproducibility.
Japanese Patent Application Laid-open Publication No. 8-56291 describes a method of “adjusting colors based on a stable image processing and process control of an input image”.
However, the change in the color reproducibility is largely influenced by color mixture of recycled toners. The color mixture causes the loss of the total color reproducibility.
Japanese Patent Application Laid-open Publication No. 2000-242152 describes a method of preventing the color mixing. Precisely, the color mixing is prevented by providing a reversibly charged toner removing unit such that the color mixture within each developing unit is effectively prevented by utilizing a difference in charge polarity between a residual toner on the image carrier after the image transfer and a reversibly transferred toner.
However, it is difficult to completely prevent the occurrence of color mixture. When the color mixture of toners within the developing unit exceeds a permissible level, there is no other way than to replace the toners using a certain method.
Japanese Patent Application Laid-open Publication No. 8-314253 describes the following method and apparatus to solve this problem. The apparatus has a controller that controls a recording unit as follows. When a developer is stirred for a long time in a state that toner in a developer container part of the developing unit is not consumed or the developer container part is not replenished with toner, the controller forms a latent image to compulsively consume toner, develops the latent image, adheres the toner onto a photoreceptor drum, and compulsively replenishes the developer container part with toner. Based on this arrangement, the apparatus can suppress degradation of the developer due to stirred friction.
However, according to this method, it is necessary to consume the toner, which requires time and labor, regardless of whether this work is carried out manually or automatically.